Multigrade lubricating oils must operate in the presence of high levels of sludge and soot resulting from oxidation during use. This accumulation of sludge and soot can seriously impair the efficiency of the oil and can result in damage to components of an engine.
To avoid these problems the multigrade lubricating oils may be formulated with dispersant additives. Various organo-metallic additives have been used previously as dispersants, but it has been recognised that the use of these dispersants may result in the deposit of metal oxides on spark plugs and so may affect engine ignition.
In addition to dispersant additives, a typical multigrade oil may also comprise viscosity index (VI) improvers. These additives are intended to produce a balance between the maximum low and high temperature viscosities. Examples of such additives are disclosed in EP-A-0331397. The additives are described as nitrogen or ester-containing adducts which are post-reacted with at least one polyanhydride. The polyanhydride couples two or more molecules of the adduct, resulting in larger polymers which are more shear-sensitive and which contribute to the high temperature viscosity to a greater extent than the low temperature viscosity when compared to additives which have not undergone post-reaction.
The additives disclosed in EP-A-0331397 are claimed to improve the viscosity characteristics in an oil, and are intended to provide a balance in the low and high temperature viscosities of the oil. The dispersancy characteristics of the additives are not investigated. What is also clear is that the additives are formed by post-reactions with relatively high levels of polyanhydride, and that this is necessary to produce highly cross-linked molecules.
Similar additives to those disclosed in EP-A-0331397 are disclosed in U.S. Pat. No. 4,747,964, where the additives are tested for their dispersancy characteristics. However, the amount of polyanhydride used to produce the additives is again relatively high, being on average 3.6% w/w.